To add stars to the background, create a new layer underneath the planet layer and fill it with black. Then go to Filter » Noise » Add Noise.

Amount: 30%
Distribution: Gaussian
Monochromatic: Checked

Now set your foreground color to black. (Press D).

then choose the Paint Bucket tool , and click on the Noise layer to paint it with black. The number of stars depends on where you click, so if you dont like it, just undo and click elsewhere until you get a nice even star field. Or you can even click again to get rid of more stars.